| Baía de São Marcos | |
| Coordinates: | -2.6°N -72°W |
| Type: | bay, estuary |
| Oceans: | Atlantic Ocean |
| Basin Countries: | Brazil |
| Length: | 100km (100miles) |
| Width: | 16km (10miles) |
| Pushpin Map: | Brazil |
The Baía de São Marcos is a bay of the Atlantic Ocean in Maranhão state of northeastern Brazil.
The bay is an estuary approximately 100km (100miles) long and up to 16km (10miles) wide. It receives several rivers, including the Grajaú, Mearim, and Pindaré. The Mearim is known for its pororoca, or tidal bore.
São Luís Island, also known as Maranhão Island, separates the Baía de São Marcos from the Baía de São Jose just to the east. São Luís Island is home to São Luís, Maranhão's capital.